Temperature gradient creates
a potential difference; connecting the
2 ends via an electrical conductor results in a current flow.
Thermoelectrics are materials which are capable of converting heat into electrical energy and vice versa. This fascinating phenomenon is nowadays commercially used in power generators (e.g., in the telecommunication industry, or in spacecrafts), food refrigerators, air conditioning, cryotherapy, pacemakers, and sensors (e.g. thermocouples). The automobile industry is eager to use this technique, e.g. for environmentally harmless air conditioning or as a power source for the radio or headlights, driven by the exhaust heat. The applications are to date limited due to the somewhat low efficiency η (ca. 5 - 10 %):
Typical values might be TH = 800 °C, TC = 400 °C, ZT = 1, yielding a theoretical η = 7.6 %.
Thermoelectrics are evaluated based on their thermoelectric figure-of-merit ZT, which is close to 1 in the materials commercially used. The Seebeck coefficient S and the thermal conductivity can be measured in our group using the commercial ZEM-3 M-8 (ULVAC-RIKO).
An increase of ZT
by a factor of two or more would be necessary to become competitive
to Freon compressors as used in conventional refrigerators.
The best thermoelectrics exhibit an intermediate charge carrier concentration (e.g., small band-gap semiconductors), a high mobility of the charge carriers (achieved by using elements with similar electronegativities) and low thermal conductivity, which can be reached by using heavy elements, mixed occupancies, rattling of atoms, and low-symmetry structures.
To improve on these characteristics is the ultimate goal in our research group.
Optimization of the Seebeck coefficient by changing the charge carrier
concentration (own research)
Temperature dependence of the thermoelectric figure-of-merit of new materials (own research)